The following described method and apparatus relates to the algal production technology which was conceived and developed over a period of about 30 years and patented as U.S. Pat. No. 4,333,263, issued Jun. 8, 1982; U.S. Pat. No. 4,966,096, issued Oct. 30, 1990; U.S. Pat. No. 5,097,795, issued Mar. 24, 1992; U.S. Pat. No. 5,851,398, issued Dec. 22, 1998; and U.S. Pat. No. 5,715,774, issued Feb. 10, 1998, the disclosures of which are incorporated herein by reference in their entireties. This apparatus is presented to satisfy a need for efficient harvest and retention of algal biomass produced on algal production systems.
Existing large-scale algal production systems marketed under the brand Algal Turf Scrubber®, or ATS systems, include in-ground troughs or “floways” in which algae is grown. These systems utilize a base of compacted soil, lined with impermeable geomembrane sheets.
Normal operation of an algal production system requires regular removal or “harvesting” of the algae, which has historically been done by scraping or vacuuming. In one process, harvesting is conducted by a tractor-type vehicle pushing a plow, and removed algal biomass is transported by water flow down the floway to a collection system comprising a chain-operated rake, filter, and containment in a sump pond for the finer material. Additional treatment can include secondary fine filtration with 10 micron mesh, but this is expensive at large scale. The coarser algae is piled onto a concrete pad for composting after removal from the water flow. The finer algae, mostly diatoms, in the sump ponds can be occasionally harvested as a fertilizer product.
Another method of harvest includes a mobile reel mechanism onto which the algal growth screen is wound. Algal biomass is scraped off the screen. The scraping operation is inefficient in terms of algal biomass recovery, and has been applied to in-ground algal production systems only. Significant damaged algal material remains on the screen to be washed away upon return to the floway, or dropped to the ground on reel-up.
Another method of harvest uses a vacuum harvest system that includes a vacuum suction unit, a generator, and container riding on a heavy cart. The cart runs on concrete side rails along an in-ground algal production system, while the vacuum suction is used to vacuum up the algae. This system requires more concrete work than the regular in-ground algal production system. Many smaller test ATS units have been harvested with portable hand-held vacuums, but this method is labor-intensive and not economical for large scale operations.
Historically, the removed algal biomass has been assigned a low economic value and has been discarded, or rendered into fertilizer or fish or other animal food. Due to the economic benefit of utilizing the algal biomass to offset costs of operation, future harvests from ATS, and other algal production systems may be retained for further processing into valuable product. These historic applications of ATS have been primarily aimed at water purification, but new uses for algal biomass such as for biofuels or pharmaceuticals require more efficient harvest and retention of algae in suitable quantity and quality for the specific purposes. In particular, effective harvesting of algal biomass requires maintaining a certain amount of moisture content in the biomass, but excess water may be detrimental to processing, necessitating a mechanism of removal whereby the moisture level is controllable.